Anaesthetic vaporisers are well known to the art, and a large number of different methods have been described. With regard to known and used vaporisers, reference is made to Anaesthetic Equipment, C. S. Ward, published by Bailliere Tindall, 2nd edition, 1987, pp. 78-103 and to Anaesthesia Vaporisers by J. B. Eisenkraft in Anaesthesia Equipment, Principles and Applications, by Jan Ekrenwerth, James B. Eisenkraft, published by Mosby, 1993, pp. 57-58.
The earlier described vaporisers are based on the principle of storing liquid anaesthetic in a container into which there is introduced a breathing gas which passes over the liquid surface or bubbles through the liquid anaesthetic.
During this passage of the breathing gas, part of the anaesthetic is vaporised and entrained by the breathing gas to the patient. This method, however, is encumbered with a large number of problems.
1. As the anaesthetic is vaporised, energy is taken from the liquefied gas, which is therewith cooled. This cooling can result in a change in the vapour pressure above the surface of the liquid and therewith also change the amount of anaesthetic that is entrained by the breathing gas.
This problem has been dealt with by delivering additional heat in the case of some designs, or by varying the amount of breathing gas that passes over the liquid surface and then combining different gas flows so as to enable a constant anaesthetic content to be obtained in the breathing gas.
2. Vaporisation of the anaesthetic is dependent on the rate of flow of the breathing gas. Attempts to compensate for this dependency have been made by using different intricate flow-dependent valves and gas mixing systems in the vaporiser. The flow dependency can become problematic, particularly in the case of low fresh-gas flows that are used in so-called low flow systems.
3. Different anaesthetics have different vaporisation characteristics and need to be used in different concentrations for optimum anaesthesia. Attempts to compensate for this have been made by designing vaporisers that are each adapted for use with solely one anaesthetic. One drawback with this resides in the risk of filling a vaporiser with the wrong anaesthetic, i.e. with an anaesthetic for which it is not intended. This would have a catastrophic effect. The need for several different vaporisers to be mounted together on a single anaesthetic apparatus also involves the risk of all vaporisers being in operation simultaneously, with the accompanying risk of administering an anaesthetic overdose.
4. Anaesthetics have different vaporisation characteristics in different gas mixtures. This can result in administering to a patient a different amount of anaesthetic than that for which the vaporiser is set, due to the composition of the gas mixture.
5. A number of systems are based on the immersion of a wick in the anaesthetic. The anaesthetic is drawn up by the wick and vaporised on its surface. The drawback with this system, however, is that the rate at which the anaesthetic is drawn up the wick will depend on the height and temperature of the liquid surface, therewith necessitating the inclusion of a compensatory system in the vaporiser.
DE-A 4 105 163 teaches a anaesthetic vaporising system in which a porous body throughpassed by anaesthetic gases is saturated with anaesthetic.
The drawback with this system is that the amount of anaesthetic that shall be used is restricted by the absorbency of said body, and that evaporation of the anaesthetic in the passing gas will vary with time, due to lowering of the temperature of said body (due to evaporation of the gas). This means that a separate temperature control circuit must be provided in order for the system to function satisfactorily. There is no pump or active means for supplying liquefied gaseous anaesthetic to the absorption-desorption material.
U.S. Pat. No. 4,015,599 describes that the absorbent keeps anaesthetic in a two-dimensional state (it is not disclosed what is actually meant by this). The anaesthetic is kept in a liquid state by means of a wick. This system also utilises a pre-charged absorbent bed through which gases pass. The drawback with this system is that it also requires the use of a temperature control means and that different evaporation-absorption rates are obtained with different anaesthetic gases.
U.S. Pat. No. 3,540,445 describes a vaporiser in which fibrous wicks have been replaced with porous synthetic plastics that absorb the anaesthetic from a container through the medium of capillary forces. Although the container can admittedly be topped-up, the amount of anaesthetic taken up by the passing gas is primarily determined by the evaporation from the porous plastic rods and the capillary forces within these rods (when the level in a vessel filled with anaesthetic is kept constant), and consequently the apparatus becomes temperature-dependent and also dependent on the anaesthetic to be vaporised.
GB 2 255 912 describes a system that uses porous rods through which the gas passes on the one hand and which are passed by the gas on the other hand. These rods are supplied with gaseous anaesthetic, by submerging the rods in the anaesthetic. The level of anaesthetic in relation to the rods is regulated by a level regulator. It is necessary to regulate the rods and the temperature of the anaesthetic and the gas in order to obtain a stable concentration of anaesthetic in the gas.
GB 2 279 015 describes an apparatus in which the liquid to be vaporised is exposed to the gas, partly through porosities and partly through the free liquid surface, thereby also requiring the provision of temperature control means. The apparatus has no liquid quantity control facility.